This invention relates to a mine support suitable for use in an underground excavation to provide support between a footwall and an opposed hanging wall.
Large excavations of the type encountered, for example, in coal mines wherein coal seams are removed using mechanised methods such as continuous coal miners, require substantial supports. These supports may for example be constructed by stacking a large quantity of wooden or concrete slabs one over the other to form a pack. Although this approach can work it is tedious and expensive. For example if the excavation height is in excess of, say, 3 m a large number of slabs must be stacked on one another to obtain the required height and, moreover, the cross sectional area of the stack must be sufficiently large to ensure that the height to area ratio of the stack is acceptable and stable under the operating conditions.
In an alternative approach to the problem it is known to make use of a tubular column which is completely filled with a yieldable material. The column is placed in position, in the excavation, using a suitable machine. This approach substantially reduces the amount of time needed to provide a support. On the other hand the characteristics of the yieldable material are such that, again, the cross sectional area of the support must be significant in order to achieve an acceptable height to cross sectional area ratio for the column. For example with a column height of the order of 3.5 m a column diameter of about 900 mm is required. Thus it is necessary to take careful consideration of the size of the column and its cost before making a decision to use a column of this type.
Similar problems and considerations present themselves when providing support in areas wherein elongate supports such as timber or steel props are used in that if the slenderness ratio of the prop (the ratio of its length or height to its cross sectional area) is too large, the prop may fail catastrophically, eg. by bending or snapping, under load instead of gradually yielding in a controlled manner.